This invention relates to a method for detecting the deformation of objects, in which during the deformation of the object a sequence of pictures of the object is taken by means of a measurement method and phase images are determined from the pictures. The sequence of pictures of the object consists of at least two pictures. The invention furthermore relates to an apparatus for performing such method.
Methods and apparatuses of this kind, in which a phase image evaluation is employed, are used for various purposes. They are employed in methods and apparatuses for detecting the shape of objects or for vibration analysis. The methods and apparatuses can employ a projection of structured light, in particular a strip projection, or an illumination with coherent light, in particular laser light. They can be used in machine tool construction, in order to detect for instance the actual deflection on a bending machine or the strain of a specimen on a tensile testing machine. Further applications are found in machines and devices for detecting surface defects or surface anomalies such as dents, depressions, scratches and the like.
A method and an apparatus as mentioned above are used in particular in the non-destructive testing of materials, especially of composite materials, and in the testing of tires (vehicle tires). In doing so, an object, such as a material, composite, workpiece, tire or the like, is deformed, which can be effected by pressure, vacuum, heat or in some other way. The object is measured in various states of deformation. By means of the measurements it can be detected whether the object includes a defect, in particular a void.
For testing tires, the tire can be placed in a vacuum chamber. The tire can be mounted on a wheel rim or can be placed in the vacuum chamber without wheel rim. When the pressure is decreased, the air enclosed in a void of the tire leads to a local expansion, which can be detected by the measurement.
A method for measuring the surface of a three-dimensional body is known from EP 419 936 B1. According to this method, an object can be irradiated with coherent radiation, in particular laser light. The reflected radiation is imaged by an imaging optics in an image plane in which a sheet-like sensor or an image sensor is provided, preferably a CCD sensor. A reference radiation with a carrier frequency is superimposed on the sensor. The imaging optics is formed or adjusted such that the image of a speckle produced on the body by the coherent radiation covers at least three sensor elements (pixels) in the image plane. Thereby, it is ensured that a complete phase measurement is possible by taking a single picture. The phase of the radiation from the object is determined from the intensity signals of the sensor elements.
In the prior art method for detecting the deformation of objects, difficulties may arise when disturbances are superimposed on the defect-typical deformation. In this case, the distances between various interference lines can become very small, so that they can be distinguished only with difficulties or not at all, whereby the evaluation is made difficult or impossible.
From U.S. Pat. No. 5,467,184 there is known a method for detecting the deformation of objects on the basis of the speckle interferometry, in which speckle pictures are taken during various states of deformation of the object and stored. To be able to also detect large deformations, speckle pictures in various states of deformation are combined into groups. The difference between the first and the last picture of each group is formed, and these differences are summed up.
EP 1 215 465 A1 discloses a method and an apparatus as stated above, which provide for a reliable evaluation even with larger deformations.